This invention pertains to the field of wire element fuses, and more particularly to low-to-medium-amperage wire element fuses and methods of making same.
Wire element fuses include a hollow fiber or other insulative housing, normally having a cylindrical shape, with conductive metal ferrules connected to the opposed ends of the housing. A fuse element or wire is strung between the opposed ferrules and held in place with a solder junction between the wire and the opposed ferrules. The amperage rating of the fuse is related to the diameter of the fuse wire. The smaller the diameter of the fuse wire, the less current which will flow therethrough before causing the wire to melt open, thereby opening the electrical circuit and thus isolating the faulty components and protecting the connected equipment from damage. The fuse opening rating is based on the amperage which is sufficient to open the fuse and is established by its characteristics. Under normal operating conditions, the fuse should carry its rated current uninterrupted.
Very small amperage fuses rated to continuously carry one-half to one-sixteenth of an amp typically use fuse wires with diameters from about 0.0015 to 0.00035 inches. To manufacture fuses with these small wires, end ferrules having small holes therein are crimped, press-fit, glued, or mechanically held by other suitable means to the ends of the housing. The fuse element is then threaded on a needle through the hole in one ferrule, through the insulative housing, and through the hole in the other opposed ferrule, and soldered by hand to the outside of the end ferrule. This procedure is followed because the fuse element has insufficient rigidity to span the insulative housing unless the fuse element is held in tension.
Larger fuses, designed to continuously carry one-half ampere and above, use fuse wires with diameters of about 0.0015 inches and higher. To manufacture fuses in this amperage rating range, a pair of washers are employed which are located on the ends of the fuse housing. The first washer is placed on the end of the housing, and a fuse wire is placed through the inner diameter of the washer until it extends out the other end of the housing. The fuse wire is bent over the top of the first washer and over the side of the fuse housing. An end ferrule is then forced over the end of the housing, washer, and fuse wire. Interference between the fuse wire, housing, and end ferrule helps maintain the ferrule in place. Arc quenching fillers which include but are not limited to materials such as silica, calcium carbonates, aluminum sulfates, and calcium sulfates are placed in the housing. A washer is then placed over the fuse wire extending out of the other end of the housing, and the extending fuse wire is bent over the washer and end of the housing. A ferrule is then interferingly engaged over the washer, fuse wire, and housing end. The assembly is then heated to reflow the solder which is pre-melted into the ferrules and create a solder junction between the ferrules and the fuse wire.
The manufacture of fuses is expensive and labor intensive. The threading and soldering operations are typically performed by hand, which results in an expensive product. Likewise, the use of washers in the manufacture of fuses is time consuming, as numerous manufacturing steps must be performed on the wire. Further, the small size of the wire results in a small contact area for the wire-ferrule solder interface, which may lead to detrimental fuse performance.